It is well known that the aluminum content of a zeolite determines to a great extent its properties such as catalytic activity, sorption and ion-exchange capacity. It is also known that acids such as strong mineral acids can be used to modify crystalline aluminosilicate powders through decationization and dealumination. It is further known that ammonium compounds have been used to convert crystalline aluminosilicates from alkali and/or alkaline metal cation form to the ammonium form, which upon calcination produces the hydrogen form.
U.S. Pat. No. 3,475,345 discloses a method of converting aluminosilicate zeolites to the hydrogen form utilizing a three-step treatment of the powdered zeolite which consists of: 1) a hot acid treatment, 2) a cold acid treatment and 3) treatment with an ammonium compound.
U.S. Pat. No. 3,442,794 discloses a method for pretreatment of aluminosilicates in the hydrogen form which is a separately performed two-step treatment with an acid compound and an ammonium compound.
Treatment of the aluminosilicates with acids has not only been effective for conversion to the hydrogen form, but also has been used as a means for increasing the silica to alumina ratio. For example, U.S. Pat. No. 3,597,155 teaches that if a mordenite powder is subjected to an acid treatment, an increase in the silica to alumina ratio is effected.
U.S. Pat. No. 3,507,931 teaches that a silica to alumina ratio above about 20:1 significantly improves the isomerization of light hydrocarbons and U.S. Pat. No. 4,018,711 teaches that isomerization performance is enhanced when a pretreated mordenite having a silica to alumina ratio of at least 19:1 is incorporated in a catalytic composition. U.S. Pat. No. 4,665,272 teaches the superior isomerization performance of a catalyst composite having a surface area of at least 580 m.sup.2 /g, as determined by the Langmuir isotherm technique, and which is approximately the same as 549 m.sup.2 /g if measured by the BET method.
The present invention provides a convenient and inexpensive method for concurrently decreasing the framework aluminum content of sodium zeolites and ion exchanging the sodium ions for ammonium and hydrogen ions to form an ammonium hydrogen zeolite.
It has been found that the addition of ammonium nitrate to nitric acid during dealumination of a zeolite dramatically improves the paraffin hydroisomerization activities of finished catalysts.